The present invention relates to the recording and reproduction of digital signals and to compensation techniques enhancing such recording and reproduction.
The art of digital magnetic recording, particularly on flexible media, has become increasingly complicated as continuing advances have been achieved both in linear densities and track densities. A recording channel engineer is faced now with important problems in a number of interacting areas.
Intersymbol interference is a problem which can be essentially ignored at lower linear densities. Such interference sometimes can be avoided by exercising design constraints on medium thickness, separation, and transducer gap lengths, as well as by using compromises between increasing densities and increased intersymbol interference. In flexible media, substrate asperities require relatively thick magnetic coatings. This problem can be contrasted with rigid disks wherein the substrate asperities are removed, allowing relatively thin magnetic coatings which tend to reduce intersymbol interference. However, such not being the case for flexible media, it is better to deal with the intersymbol interference than avoid it. These techniques then require equalization components to compensate for head to medium separation losses, gap loss in the transducer, and generally the lack of DC and low frequency response as well as phase or peak shift during recording.
Such equalization is largely determined by the clocking and detection methods. Threshold level detection and peak detection were the earlier methods used and are widely used despite peak shift and amplitude variation problems. Sampling detectors which drive a clock from zero crossings and then sample at the middle of bit period are often used with the flexible media, as well as with the rigid disk media. In turn, detection strategy is influenced by the recording system structure. The detection has to accommodate the dominate forms of interfering noise. When random noise is a major problem, then integration detectors are more useful. In contrast, when the readback signal is subject to severe amplitude variations, sampling detectors are more useful. The latter is suitable for flexible media wherein the media defects and so-called dropouts essentially determine the error rate of the data stream. That is, errors almost never occur under nominal good media conditions. Now as track densities increase, track following errors in disk environments lead to interference from adjacent record tracks as well as from old data in the present track--i.e., synchronous noise. These noise sources have characteristics similar to the desired data, such that integration detectors and simple peak or amplitude detectors cannot always successfully distinguish signals from noise. The combination of all of these effects poses severe problems in the design and manufacture of signal detection systems for magnetic recorders using flexible media.
Sampling detectors are useful in a recording channel having closely equalized raised cosine pulse spectrum. In general, magnetic recording channels cannot faithfully reproduce the DC and long wavelengths required for true or high quality raised cosine spectrum. Therefore, usually data encoding is used to reduce the long wavelength signal energy and thus control the wandering signal base line. Such encoding reduces the data rate, reduces detection capabilities and requires encode-decode circuits.
Flexible disk media exhibits anisotropic dimensional changes with temperature, humidity and age. These changes differ irregularly between batch sources of media, as well as locally within a given disk, card or tape. Track following of the servo-controlled type on an individual track basis appears to be an absolute necessity for increasing track density. Areal density is chiefly limited by track density, which in turn is determined by achievable track-following accuracy in combination with achievable off-track tolerances of data detection circuits. Such problems are complicated by different speeds of the media with respect to transducers. Accordingly, it is desired to have a control signal embedded in the record media wherein the record media is but a single magnetic coating layer. If a control signal is recorded before data signals are recorded, then the repeated recording and reproductions of data signals should not interfere with the quality of the control signal in the media. Therefore, it is desired to select a recording technique together with compensation techniques that will accommodate the preservation of prerecorded control signals.
The response of a magnetic recording channel diminishes rapidly at low densities and is zero at DC or zero frequency. Such low frequency response is limited by the finite length and height dimension of the transducer pole tips, by the finite medium thickness, and by the limited recording depth. Commonly used detection systems operate on a bit-by-bit technique; detection of the presence or absence of recording transitions occurs one at a time. The only information used is that of the transitions. Presently, low frequency components of the data signal are recorded onto the media even if they cannot be faithfully reproduced or successfully used in the data detection process. Therefore, the absence of such low frequencies should be facility accommodated.